There is a growing interest in electronic color imaging for technologies that are dry processable and environmentally friendly. Among them, direct thermal, thermal mass transfer, thermal dye transfer, and dry silver imaging are the most practical and well-received processes. In all of these processes, heat is the main driving force that generates visible images. Therefore, these imaging processes are strongly dependent on how efficient the heat is utilized for imaging without unnecessary heat losses to the surrounding areas.
In thermal dye transfer imaging, an image is formed on a receptor sheet by selectively transferring a dye to a receptor sheet from a dye donor element (sheet or ribbon) placed in intimate contact with the receptor sheet. Material (e.g., dye) is transferred from the donor element, e.g., the ribbon, upon localized heating (such as that directed by a thermal print head, which consists of small electrically heated elements [print heads]). Heat generated momentarily from these elements is transferred through the dye donor to the dye receiving layer and the base of the receptor in several milliseconds. The thermal energy is used to heat and mobilize the dye, to soften the interpolymeric cohesion, and to open up free space in the resinous binders used in the donor dye layer and dye receiving layer allowing passage and "parking room" for the traveling dye molecules. Temperature plays a critical role here; the higher the temperature, the higher the dye image density.
Thermal dye transfer systems in general have advantages over thermal mass transfer system in that the former is capable of producing high quality, continuous tone, full color images. However, the systems suffer from two major drawbacks that impair their ability to make truly photographic quality color hardcopies. One of the drawbacks is that the systems need very high transfer energies, as high as several Joules of heat per square centimeter, to produce an image with adequate color density. This drawback is particularly serious in the case of making transparencies for medical imaging and overhead visual projection where high transmission optical densities are required but are difficult to achieve. In some cases, a single pass printing may not be able to produce enough image density and multiple-pass printings, as disclosed in U.S. Pat. No. 4,833,124, may be needed. The other drawback is that the dye image quality, as measured by color uniformity and density, is very much limited by the receptor's properties.
These two problems are well known in the industry, and attempts to solve them are being made by others using different approaches. For examples, U.S. Pat. Nos. 4,734,396 and 4,734,397 teach the use of a non-porous compression layer (with a compression modulus less than 350 mega Pascal) coated on a substrate to improve image-quality. The compression layer is, for example, poly(methylmethacrylate), poly(styrene-co-acrylonitrile), or polyurethane. U.S. Pat. No. 4,912,085 discloses the use of a receptor substrate comprising a molecularly oriented film of a synthetic thermoplastic polymer and an inorganic filler (i.e., polyester with barium sulfate). U.S. Pat. No. 4,935,402 also discloses the use of a dye receptor substrate made of an extruded, biaxially stretched sheet of a mixture of white particles and a polyester resin. U.S. Pat. Nos. 4,778,782 and 4,971,950 separately disclose the use of a synthetic paper (made of polyolefin resin and inorganic pigment, i.e. CaCO.sub.3),as a dye receptor substrate. The polyolefin film was biaxially oriented to create microvoids.
The present invention overcomes the drawbacks of thermal dye transfer systems by providing the receptor with a uniform, microporous thermal insulating polymeric film. The films when properly incorporated in the receptors as described in this invention effectively reduce the energy requirements for dye transfer and significantly improve the resulting image uniformity and densities. The films are opaque white but transparentizable because they contain essentially no inorganic pigments in the thermoplastic films (i.e., no significantly visible amount, having an optical density of less than 0.20 D. at a thickness of 0.05 mm). This allows the production of transparencies as well as non-transparency prints.